Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes, either on labels or directly on articles to be encoded. The thermal transfer process employed by these printing methods provides great flexibility in generating images allowing for broad variations in the style, size and color of the printed images, typically from a single machine with a single thermal print head.
Representative documentation in the area of thermal transfer printing includes the following patents:
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, which discloses a thermal transfer medium having a coating composition of cellulosic polymer, thermoplastic resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,523,207, issued to M. W. Lewis et al. on Jun. 11, 1985, discloses a multiple copy thermal record sheet which uses crystal violet lactone and a phenolic resin.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,698,268, issued to S. Ueyama on Oct. 6, 1987, discloses a heat resistant substrate and a heat-sensitive transferring ink layer. An overcoat layer may be formed on the ink layer.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al. on Nov. 17,1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filling layer laminated on the ink layer.
U.S. Pat. No. 4,865,901, issued to Ohno et al. on Sep. 12,1989, discloses a thermal transfer printing ribbon with an ink layer comprising a blend of ethylene-vinyl acetate copolymer and a viscous resin as a binder with correction/erasability capabilities.
U.S. Pat. No. 4,869,941, issued to Ohki on Sep. 26,1989, discloses an imaged substrate with a protective layer laminated on the imaged surface.
U.S. Pat. No. 4,894,283, issued to Wehr on Jan. 16,1990, discloses a reusable thermal transfer ribbon with a functional layer and a binding layer containing 100% ethylene vinyl acetate copolymer.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8,1990, discloses a thermal transfer ribbon which comprises two layers, a thermosensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4,1990, discloses a recording medium for transfer printing comprising a base film, an adhesiveness improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8,1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29,1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
U.S. Pat. No. 5,240,781, issued to Obata et al. on Aug. 31,1993, discloses an ink ribbon for thermal transfer printers having an ink layer with viscosity, softening and solidifying characteristics said to provide clear images on rough paper even with high speed printers.
As the use of thermal transfer printing grows into new applications, the requirements for the ribbons become broader and more strict. For example, providing print with smudge and scratch resistance, chemical resistance and suitability for rough stock (receiving substrates) can require special formulations for the thermal transfer media. The use of printers having heating elements displaced right at the edge of the print head has been favored in that this configuration extends the life of the print head. Such printers are known in the art as "near edge", "true edge" and "feather edge" printers and are referred to herein collectively as "high speed printers" due to the rapid separation of the ribbon from the substrate once the print head heating elements have been fired.
With the advent of high speed printers, modification of conventional thermal transfer ribbons has been found to be necessary. Conventional thermal transfer ribbons do not perform satisfactorily with high speed printers in that the ribbon and receiving substrate are separated almost spontaneously after the thin film resistors are fired and there is very little time for waxes and/or resins to melt/soften and flow onto the surface of the receiving substrate before the ribbon is separated from the receiving substrate. With conventional ribbons, the adhesion of the molten/softened material to the receiving substrate is typically lower than its adhesion to the supporting substrate of the ribbon at the time of separation with a high speed printer. As a result, the functioning thermal transfer layer is usually split and the transfer incomplete, resulting in light printed images where the functioning layer is an ink layer.
The use of an adhesive layer, comprising polycaprolactone and no pigment, on top of a functioning layer is disclosed by Obata et al. in U.S. Pat. No. 5,240,781. Such a configuration has been found not to provide the best offset resistance and darkest density of printed images for other formulations. A new configuration is desired for thermal transfer ribbons which do not require a polycaprolactone adhesive layer and which is more resistant to offset and provides dark density images.